Silica fume (SF) is a byproduct of the induction arc furnace process in the silicon metal and ferrosilicon alloy industries. At temperatures of 2000°C, quartz is reduced to silicon while generating silicon vapor, which is oxidized and condensed into fine spherical particles (with an average diameter of approximately 0.1μm) in the cooler regions. These particles consist of amorphous silica (>85%) and possess high pozzolanic activity. Silica fume can be collected as a powder through dust collection devices.
In Ultra-High Performance Concrete (UHPC), the influence of Silica fume on cement hydration, microstructural evolution, and strength development, without considering the workability of the mixture, is primarily divided into physical and chemical effects.
1. Physical Effects-Filler Effect
The filler effect, which arises from the physical presence of particulate matter, leads to changes in the cement hydration process and the microstructure. It can be broken down into several aspects:
Dilution Effect: As Silica fume replaces a portion of the cement, it increases the effective water-to-cement ratio at the same water-to-binder ratio, allowing more water to participate in cement hydration and providing more space for the growth of hydration products.
Nucleation Effect: The smaller particles, due to their larger specific surface area, can provide nucleation sites for cement hydration products, promoting the hydration of cement.
Packing Effect: The extremely fine Silica fume particles can displace water in smaller spaces within the concrete, resulting in a denser concrete matrix.
It should be noted that the mechanisms of the filler effect are largely similar across various sources. I have synthesized these explanations into the aforementioned three aspects: physical effects, nucleation effects, and packing effects. These effects are all part of the filler effect, distinct from the pozzolanic reaction.
2. Chemical Effects - Pozzolanic Reaction
Generally speaking, Supplementary Cementitious Materials (SCMs) contain the same chemical elements as the clinker in Portland cement, but their proportions and combinations in the solid phase are different. Therefore, as long as calcium hydroxide (CH) is available, SCMs will react in the concrete and produce similar hydration products. However, the proportion and composition of these hydration products can change significantly with variations in the amount of SCMs added. The pozzolanic reaction of Silica fume forms C-S-H: S + 1.7CH + 2.3H → C1.7SH4
Hydration is the process where cement particles come into contact with water, and the clinker minerals react with water to undergo hydration, transforming from an anhydrous state to a hydrated state with bound water. This process is a very complex series of physical, chemical, and physicochemical changes. The introduction of Silica fume leads to a more complex cementitious system: The hydration reaction of cement and the pozzolanic reaction of Silica fume occur simultaneously and may affect each other's reactivity; the presence of Silica fume affects the quantity and type of hydration products formed in the cementitious system, thereby influencing its volume, porosity, and ultimate durability; the kinetics of the pozzolanic reaction of Silica fume depend on its chemical composition, fineness, glass phase content, and the composition of the pore solution.
